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One of the most common errors in the English language is the one made in the use of which and that when introducing clauses that modify nouns. Even the best of writers often stop to think – should I use a which here, or a that?

A restrictive clause is a clause without which the meaning of a sentence would change. For example:

I liked the purse that was in the display window.

The clause here is ‘that was in the display window’. If we remove this clause, the meaning of the sentence will change. Why? Because then we wouldn’t know which purse the writer is talking about. The clause here is restricting, or limiting, the scope of the purse – meaning that the writer doesn’t mean just any purse, but that one particular purse in the window.

A non-restrictive clause is a clause without which the meaning of a sentence will not change. In other words, the clause is only giving additional information. For example (taking the same sentence):

I liked the purse, which was in the display window.

Now the clause is ‘which was in the display window’. Here we know which purse the writer is talking about, and the clause is just giving us additional information – as if to say, “by the way, the purse was in the display window”. So, if we are to remove the clause, we will still know which purse the writer is referring to. .

Now you see why there is so much confusion between the use of which and that? The difference between the two is very slight, and unless care is taken, or rather the difference between restrictive and non-restrictive clauses is clearly understood, mistakes will be made. So, do be very careful if you want your writing to be perfect.

James Thurber once said, “What most people don’t realize is that one which leads to another.” What do you think?